A storm expected to bring more than a foot of snow, stiff winds and punishing cold pushed into the Northeast on Thursday, extending Christmas break for some students while posing the first test for New York's new mayor and perhaps the last challenge for Boston's outgoing one. Some schools in New England and New York closed well ahead of the snow, while cities mobilized plows and salt spreaders, and state offices sent workers home early. Some major highways were ordered shut down overnight. U.S. airlines canceled more than 2,300 flights nationwide on Thursday in advance of the storm.